A rare celestial event was captured by NASA's Chandra X-ray Observatory
as Titan -- Saturn's largest moon and the only moon in the Solar System
with a thick atmosphere -- crossed in front of the X-ray bright Crab
Nebula. The X-ray shadow cast by Titan allowed astronomers to make the
first X-ray measurement of the extent of its atmosphere.

On January 5, 2003, Titan transited the Crab Nebula, the remnant of a
supernova explosion that was observed to occur in the year 1054.
Although Saturn and Titan pass within a few degrees of the Crab Nebula
every 30 years, they rarely pass directly in front of it.

"This may have been the first transit of the Crab Nebula by Titan since
the birth of the Crab Nebula," said Koji Mori of Pennsylvania State
University in University Park, and lead author on an Astrophysical
Journal paper describing these results. "The next similar conjunction
will take place in the year 2267, so this was truly a once in a lifetime
event."

Animation of Titan's Shadow on Crab Nebula

Chandra's observation revealed that the diameter of the X-ray shadow
cast by Titan was larger than the diameter of its solid surface. The
difference in diameters gives a measurement of about 550 miles (880
kilometers) for the height of the X-ray absorbing region of Titan's
atmosphere. The extent of the upper atmosphere is consistent with, or
slightly (10-15%) larger, than that implied by Voyager I observations
made at radio, infrared, and ultraviolet wavelengths in 1980.

"Saturn was about 5% closer to the Sun in 2003, so increased solar
heating of Titan may account for some of this atmospheric expansion,"
said Hiroshi Tsunemi of Osaka University in Japan, one of the coauthors
on the paper.

The X-ray brightness and extent of the Crab Nebula made it possible to
study the tiny X-ray shadow cast by Titan during its transit. By using
Chandra to precisely track Titan's position, astronomers were able to
measure a shadow one arcsecond in diameter, which corresponds to the
size of a dime as viewed from about two and a half miles.

Illustration of Crab, Titan's Shadow and Chandra

Unlike almost all of Chandra's images which are made by focusing X-ray
emission from cosmic sources, Titan's X-ray shadow image was produced in
a manner similar to a medical X-ray. That is, an X-ray source (the Crab
Nebula) is used to make a shadow image (Titan and its atmosphere) that
is recorded on film (Chandra's ACIS detector).

Titan's atmosphere, which is about 95% nitrogen and 5% methane, has a
pressure near the surface that is one and a half times the Earth's sea
level pressure. Voyager I spacecraft measured the structure of Titan's
atmosphere at heights below about 300 miles (500 kilometers), and above
600 miles (1000 kilometers). Until the Chandra observations, however,
no measurements existed at heights in the range between 300 and 600
miles.

Understanding the extent of Titan's atmosphere is important for the
planners of the Cassini-Huygens mission. The Cassini-Huygens spacecraft
will reach Saturn in July of this year to begin a four-year tour of
Saturn, its rings and its moons. The tour will include close flybys of
Titan that will take Cassini as close as 600 miles, and the launching of
the Huygens probe that will land on Titan's surface.

Chandra's X-ray Shadow of Titan

"If Titan's atmosphere has really expanded, the trajectory may have to
be changed." said Tsunemi.

The paper on these results has been accepted and is expected to appear
in a June 2004 issue of The Astrophysical Journal. Other members of the
research team were Haroyoski Katayama (Osaka University), David Burrows
and Gordon Garmine (Penn State University), and Albert Metzger (JPL).
Chandra observed Titan from 9:04 to 18:46 UT on January 5, 2003, using
its Advanced CCD Imaging Spectrometer instrument.